1. Field of the Invention
The present invention relates to a semiconductor memory device and particularly to a MOS dynamic RAM.
2. Description of the Prior Art
In the prior art, the one-transistor and one-capacitor system is often adopted in a memory cell of a MOS dynamic RAM. FIG. 1 is a top view of a memory cell of a conventional MOS dynamic RAM and FIG. 2 is a cross sectional view taken along the line A--A' in FIG. 1.
First, the structure of the conventional device will be described. On a p type silicon substrate 6, a first polysilicon gate 2 is formed through a first gate oxide film 1 and a second polysilicon gate 3 is formed through a second gate oxide film 4. In addition, on the p type silicon substrate 6, an n+ type semiconductor region 5 is formed. The first polysilicon gate 2, the first gate oxide film 1 and the p type silicon substrate 6 constitute a first transistor. A capacitor is formed between a lower portion of the first polysilicon gate 2 and a channel portion just under the first gate oxide film 1. The n+ semiconductor region 5, the second polysilicon gate 3 and a source region or a drain region just under the first gate oxide film 1 constitute a second transistor. The capacitor of the first transistor and the second transistor constitute a memory cell. The capacitance of the capacitor serves as a capacitance for storing data and the second transistor is used for writing and reading a signal in the capacitor.
Now, the operation of this conventional device will be described. In the memory cell, a power supply voltage is normally applied to the first polysilicon gate 2 and the capacitor is used as a memory device. Writing of a content in the capacitor is performed in the following manner. Normally, a potential according to the content to be written, for example, a plus voltage if the content is "1" or a zero voltage if the content is "0" is applied to the n.sup.+ type semiconductor region 5 and then, a plus voltage is applied to the second polysilicon gate 3 corresponding to the gate of a writing transistor to conduct the second transistor, whereby "1" or "0" is written in the capacitor. Then, the second polysilicon gate 3 is grounded so that the content in the capacitor is maintained. Reading of the content from the capacitor is performed in the following manner. A plus voltage is applied again to the second polysilicon gate 3 to conduct the second transistor so that the potential of the n.sup.+ type semiconductor region 5 is changed according to the content of the memory capacitor. The change of the potential is amplified by a sense amplifier.
Recently, according to an increasing demand for large-scale integration and high density of memories, the area of an ordinary capacitor has been made small. For example, compared with a 16K bit RAM in which the area of a capacitor was about 400 .mu.m.sup.2, the area of a capacitor in a 64K bit RAM is about 200 .mu.m.sup.2 and that in a 256K bit RAM is about 70 .mu.m.sup.2.
In order to prevent a decrease in a capacitance due to such reduction of a capacitor area, a method for making thin a gate oxide film, for example, is often adopted. However, in view of a higher degree of integration of memories developed in future, decrease of a capacitance is considered to be unavoidable. In addition, since a capacitor of a conventional MOS dynamic RAM has a plane structure, a problem is involved that the capacitance is decreased due to the reduction of a capacitor area caused by such high degree of integration.